A diesel engine system generally comprises an intake manifold, at least one combustion chamber, an exhaust manifold, and an exhaust line provided with a diesel oxidation catalyst (DOC).
The diesel oxidation catalyst is conventionally provided for degrading residual hydrocarbons and carbon oxides, which are formed in the combustion process of the engine and are contained in the exhaust gas flow.
In order to accomplish tighter emission legislation, most of the diesel engine systems are also equipped with a diesel particulate filter (DPF), which is located in the exhaust line downstream the DOC for capturing and removing diesel particulate matter (soot) from the exhaust gas flow.
The diesel particulate filters generally comprise a casing which contains a filter body of porous material, with dead-end holes extending into the filter body from opposite sides thereof. In normal operation, exhaust gas enters the dead-end holes from one side of the filter body, and passes through the filter material into the dead-end holes of the other side, whereby the particulate matter carried by the exhaust gas is retained at the surface and in the pores of the filter body.
The accumulating particulate matter increases the pressure drop across the filter. When the pressure drop becomes excessive, it may cause the filter body to crack, rendering the filter ineffective, or it may affect the efficiency of the diesel engine.
In order to avoid excessive clogging of the filter, the particulate matter must be removed when critical amount of it has accumulated in the filter body.
This process is generally referred to as regeneration of the diesel particulate filter. Conventionally, regeneration is achieved by heating the DPF to a temperature at which the accumulated particulate matter burns off, leaving the filter body clean again.
The heating of the filter is provided by means of a temperature increase of the exhaust gases entering the DPF. This temperature increase (typically up to 630° C.) has to be kept for a certain time (typically 600 seconds) in all possible driving condition (i.e. city driving, highway driving, etc.).
Exhaust gas temperature increase is obtained with a dedicated multi-injection pattern, by means of which an amount of fuel is injected into the combustion chamber after the piston has passed its top dead center position, and the fuel that was injected before is already burnt.
Such late-injected fuel can get a first temperature increase due to fuel combustion inside combustion chamber, and a second temperature increase due to fuel oxidation inside the catalyst (DOC) of the exhaust line. More particularly, the first temperature increase is achieved by a single injection of fuel which is generally referred to as after-injection. The after injection starts before the exhaust valves opening, and sufficiently near to TDC for the fuel to burn quite completely into the combustion chamber. The combustion of after-injected fuel produces hot gases which are subsequently discharged from the combustion chamber and channeled by the exhaust line to pass through the DPF, whereby the latter is heated.
The second temperature increase is achieved by one or more injections of fuel which are generally referred to as post-injections. Post-injections start sufficiently far from TDC for the fuel to not burn into the combustion chamber, typically after the exhaust valves opening. Therefore, the post-injected fuel is ejected unborn from the combustion chamber and is channeled by the exhaust line towards the diesel oxidation catalyst (DOC).
When the particulate matter load in the DPF is high, after the appropriate regeneration temperature is reached, the burning of every single particle generates further heat, which is quite efficiently transferred to a nearby particle, causing it to burn too. When the concentration of particles in the DPF decreases, this type of heat transfer tends to become less effective. It implies that at the beginning of the regeneration process the temperature inside the DPF increases rapidly. If the combustion of the particulate matter is not controlled, said temperature increase can be faster and higher than necessary and, under certain circumstances, can also damage the diesel particulate filter.
The amount of oxygen in the exhaust gases downstream the DOC affects the combustion of particles inside the DPF, and therefore it is a key parameter in controlling temperature gradients inside the DPF during regeneration process. However, no control systems are actually available for measuring and controlling the oxygen concentration downstream the diesel oxidation catalyst, probably because the control systems which rely upon oxygen sensing technology have many drawbacks.
As a matter of fact, said control systems are generally satisfactory for managing steady state or slowly varying oxygen levels, but are not satisfactory for managing quickly varying oxygen levels which can be found at some points within the diesel engine system. Moreover, known wide range oxygen sensing technology are affected by the temperature and pressure conditions, so that they often require proper compensation to produce accurate oxygen concentration information. Besides, oxygen sensors measurement is not accurate when the sensor is waking with a high hydrocarbons concentration in the exhaust gas flow, which is the case of DPF regeneration conditions.
At least one aim of the present invention is to estimate the oxygen concentration downstream a Diesel Oxidation Catalyst (DOC) within diesel engine systems. Another aim of the present invention is to meet the goal with a rather simple, rational and inexpensive solution. In addition, other aims, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.